I am also sorry for the loss you are experiencing. There are timnes this happens. If your husband died suddenly, he could have had a stroke or a blood clot; more than likely his death was caused by a ventricular arrhythmia. An arrhythmia would not have shown up on his heart tests unless they were occuring at the same time he was having an EKG or cath done.
I'm so sorry to hear about your husband. You're right, if all tests were negative and yet it was heart related, they should do a post mortem to establish the cause of death. They have to put a reason on the death certificate.